The invention relates to a method for detecting a defective air mass flow meter for detecting the air mass flow of intake air for a number of cylinders of an internal combustion engine, in particular in a motor vehicle.
In internal combustion engines, air mass flow meters are frequently installed in the intake manifold of the engine and measure the air mass flow of the intake air. The detected air mass flow is used here within the scope of the engine control to determine the quantity of fuel which is to be apportioned to the cylinders. Soiling, thermal stresses or wear can result in deviations in the signal of the air mass flow meter, i.e. the air mass flow detected by the air mass flow meter deviates from the actual air mass flow. Such defective air mass flow meters give rise to incorrect apportioning of fuel, which over the long term causes the internal combustion engine to malfunction and results in the outputting of a warning message in the vehicle.
Mixture faults in internal combustion engines can result not only from defective air mass flow meters but also from a variety of other causes. They can, for example, be due to a faulty fuel supply system, to a faulty air supply system or to further faulty sensors. However, the air mass flow meter is often the single trigger of a mixture fault. Owing to the large number of possible causes, it is currently difficult to identify the air mass flow meter as a damaged part. In particular, there is currently no possible way of detecting the defect in an air mass flow meter in the installed state in the vehicle. In addition, there are also no tests for checking uninstalled air mass flow meters which can be carried out easily. In order to overcome mixture faults, a large number of repeated repairs are therefore often necessary, since components are exchanged on suspicion. This leads to increased costs.
The object of the invention is therefore to provide a method and a corresponding engine test device with which defective air mass flow meters can be detected easily and reliably in the entire system of the internal combustion engine without requiring the air mass flow meters present to be uninstalled.
The method according to the invention serves to detect a defective air mass flow meter, and if appropriate also a plurality of defective air mass flow meters, in an internal combustion engine. The air mass flow meter serves here to detect the air mass flow of intake air for a number of cylinders of the internal combustion engine, i.e. for a cylinder bank. The internal combustion engine is preferably the engine in a motor vehicle.
According to the invention, within the scope of a test procedure the internal combustion engine is operated by a pilot controller with a mixture controller deactivated. The pilot controller outputs here, in a manner known per se, actuating values for the mixture of fuel and air in the respective cylinders, i.e. the pilot controller defines in advance suitable values for the quantity of fuel which is to be apportioned. These values of the pilot controller are varied in a manner known per se in the normal operating mode of the internal combustion engine (i.e. not in the operating mode of the test procedure according to the invention) by the (activated) mixture controller on the basis of a mixture deviation of the current mixture from a target mixture, e.g. a stoichiometric mixture. The actuating values which are output by the pilot controller should correspond here as far as possible to the actuating values which are adapted or varied by the mixture controller. The mixture deviation is detected by a measuring device and, in particular, by a lambda probe which is known per se. In particular, the mixture deviation is represented by the lambda value or the deviation of the lambda value from 1. The lambda value here is a sufficiently known variable, wherein a lambda value of 1 represents the stoichiometric mixture. Lambda values above 1 indicate excess air and therefore a “lean” mixture. Lambda values below 1 characterize excess fuel and therefore a “rich” mixture.
In a step a) of the test procedure according to the invention, the internal combustion engine is set to one or more different idling speeds and the mixture deviation is detected for a respective idling speed with the mixture controller deactivated. According to an evaluation in a step b) the air mass flow meter is categorized as defective when the detected mixture deviation or detected mixture deviations satisfy a number of criteria (i.e. at least one criterion), wherein the first criterion from the number of criteria is satisfied when the detected mixture deviation for one of the set idling speeds exceeds a predetermined amount. In particular, the first criterion is satisfied when the detected mixture deviation for the maximum set idling speed exceeds a predetermined amount in the direction of mixtures with excess fuel with respect to the stoichiometric mixture. If the mixture deviation is detected on the basis of lambda values, in this case the measured lambda value undershoots a threshold value of less than 1.
The method according to the invention is based on the realization that defective air mass flow meters can be detected easily by analyzing mixture deviations at different idling speeds. A defective air mass flow meter is understood here to be an air mass flow meter whose measured air mass flows deviate from the actual air mass flows beyond a tolerance region. In other words, a defective air mass flow meter does not have to be completely operationally incapable but rather it is to be considered as defective if it no longer supplies sufficiently accurate measurement values.
In one particularly preferred embodiment, with the method according to the invention the specific fault pattern of an air mass flow meter is detected when the measurement values differ greatly from the actual air mass flows in the case of low air mass flows, and in this context represent, in particular, an excessively high air mass flow. In this case, the criteria described above comprise, in addition to the first criterion, also a second criterion and a third criterion, wherein only when the first, second and third criterion are satisfied is a defective air mass flow meter directly detected. In this case, the second criterion is satisfied when the detected mixture deviation with increasing set idling speeds decreases (requirement of monotony). In addition, the third criterion is satisfied when the difference in terms of absolute value between the mixture deviation when a minimum idling speed is set and that when a maximum idling speed is set exceeds a predefined amount.
As already mentioned above, the detected mixture deviation is preferably represented by a deviation of the lambda value from 1, wherein the lambda value is, in particular, a mean value of a plurality of lambda values, measured in a steady-state measuring interval, for the respective set idling speed. A steady-state measuring interval is defined here by the fact that the operating parameters of the internal combustion engine have only small fluctuations (i.e. fluctuations within corresponding tolerance ranges) in the measuring interval.
If the mixture deviation is represented by the deviation of the lambda value, the first criterion and/or second criterion and/or third criterion are preferably defined in such a way that:                the first criterion is satisfied when the lambda value with the maximum set idling speed deviates from 1 by more than a predefined threshold value, and this lambda value is also preferably less than 1;        
and/or                the second criterion is satisfied when the lambda value with increasing set idling speed values becomes larger and in this case is preferably never above 1;        
and/or                the third criterion is satisfied when the difference in terms of absolute value between lambda values with the minimum set idling speed and the maximum set idling speed exceeds a predefined difference value.        
The threshold value just defined for the first criterion is preferably between 0.08 and 0.2, in particular is 0.115. The predefined difference value just defined for the third criterion is preferably between 0.03 and 0.1, in particular is 0.06.
In a further preferred embodiment the preset idling speeds comprise one or more of the following idling speed values:                an idling speed value between 800 and 900 rpm (rpm=revolutions per minute), in particular of 850 rpm;        an idling speed value between 1000 and 1100 rpm, in particular of 1050 rpm;        an idling speed value between 1400 and 1600 rpm, in particular of 1500 rpm; and        an idling speed value between 1900 and 2100 rpm, in particular of 2000 rpm.        
In a further preferred variant of the method according to the invention, one or more operating parameters of the internal combustion engine are kept constant for the respective set idling speed. Such operating parameters relate, in particular, to the electric load at the internal combustion engine and/or the control times of valves of the internal combustion engine and/or the coolant temperature of the internal combustion engine. Nevertheless, the operating parameters can also relate to other parameters of the engine. With this variant, the reproducibility of the test according to the invention is ensured.
In a further refinement of the method according to the invention, the actuating values which are output by the pilot controller are adapted by percentage adaptation values for respective operating points of the internal combustion engine with different engine rotational speeds. In other words, the pilot controller comprises what is referred to as a mixture adaptation system, in which the actuating values of the pilot controller are adapted in a suitable way directly in the pilot controller if said values deviate continuously from the actuating values which are subsequently adjusted by the mixture controller. The corresponding adaptation values are stored here in a linking fashion in the pilot controller and as a rule are continuously learnt during the operation of the internal combustion engine. In the case of the use of such a mixture adaptation system, in one preferred variant of the invention before the execution of step a), the adaptation values for different rotational speeds are read out from the pilot controller. After the reading out of the adaptation values, they are preferably reset in the pilot controller, i.e. set to values at which the actuating values of the pilot controller are not adapted. Accordingly, the subsequent engine test is carried out with the original, non-adapted actuating values, as a result of which the reproducibility of the test is ensured. Furthermore, with this approach the test procedure is adapted to the conditions of the engine after delivery, that is to say were adapted in relation to a time at which it was not yet possible to learn any mixture adaptation values.
If read-out adaptation values are present, a number of further criteria are checked if the air mass flow meter is not categorized as defective in the above step b). The number of further criteria comprises here a criterion relating to the adaptation values, wherein in order to satisfy the criterion relating to the adaptation values it is necessary for the minimum adaptation value which is read out to undershoot a predefined first threshold and for the maximum adaptation value which is read out to exceed a predefined second threshold. If the number of further criteria are satisfied, the air mass flow meter which was originally categorized as not defective is now definitively evaluated as defective. When the number of criteria are not satisfied, the air mass flow meter continues to be categorized as intact.
In one particularly preferred embodiment, the criterion relating to the adaptation values is satisfied when the minimum adaptation value which is read out undershoots a predefined first threshold, in particular −13% of the amount of fuel to be injected, and the maximum adaptation value which is read out exceeds a predefined second threshold, in particular +1% of the amount of fuel to be injection, and in addition the rotational speed value of the internal combustion engine in the case of the minimum adaptation value is below a first rotational speed threshold, in particular below 1750 rpm, and the rotational speed value of the internal combustion engine in the case of the maximum adaptation value is above a second rotational speed threshold, in particular above 5250 rpm.
In a further preferred embodiment, when the internal combustion engine is switched off, an idle signal of the air mass flow meter is detected, wherein a variant of such an idle signal is explained in the specific description. The number of further criteria additionally comprises here a criterion relating to the idle signal which is satisfied when the idle signal is outside a standard range. An air mass flow meter is therefore categorized as defective only when both the criterion relating to the adaptation values and the criterion relating to the idle signal are satisfied.
In a further refinement of the method according to the invention, before the start of the test procedure the presence of one or more starting conditions to be satisfied by the internal combustion engine is checked, wherein the test procedure is started only when the starting condition or conditions is/are satisfied. Various variants of such starting conditions can be found, inter alia, in the specific description.
In one particularly preferred embodiment, one or more of the starting conditions are based on fault entries in the fault memory of the control device of the internal combustion engine, wherein a starting condition is preferably configured in such a way that the relative load (a measure of the quantity of air in the cylinder of the internal combustion engine) in the case of the corresponding fault entries is below a threshold value, preferably below 40%. This takes into account the fault pattern described above, according to which the air mass flow meter supplies incorrect air mass flows at low loads (i.e. real low air mass flows) and therefore gives rise to high mixture faults. The term relative load is a customary term for a person skilled in the art and is 100% in the case of induction engines with the throttle valve fully opened and 20% for operating points near to idling. If the engine comprises a turbocharger or some other supercharging method, the relative load can also be above 100%.
In addition to the method described above, the invention also relates to an engine test device for detecting a defective air mass flow meter for detecting the air mass flow of intake air for a number of cylinders of an internal combustion engine. The engine test device is configured to carry out the method according to the invention or one or more preferred variants of the method according to the invention. The engine test device can be an external engine test device here or, if appropriate, can also be integrated into the motor vehicle.
The invention furthermore relates to a motor vehicle with an internal combustion engine and one or more air mass flow meters, wherein the motor vehicle comprises the engine test device described above for testing the air mass flow meter or meters.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawings.